Problem: Fatma got $5$ tetra fish for her birthday. She has a prism-like water tank whose base area is $0.5$ square meters and height is $0.4$ meters. Assuming Fatma fills the tank in its entirety, what is the density of the fish in the tank?
Answer: This is a density word problem. To solve it, we can use the following equation, which is the volume definition of density: ${\text{Density}}=\dfrac{{\text{Total quantity}}}{{\text{Volume}}}$ What do we know? There are $ 5$ fish, which is the ${\text{total quantity}}$. The base area of the prism-like tank is $0.5$ square meters and its height is $0.4$ meters (we can use this to find the ${\text{volume}}$ ). What do we need to find? The ${\text{density}}$ of the fish in the tank. If Fatma fills the whole tank, the ${\text{volume}}$ of the water is $0.5\cdot 0.4={0.2}$ cubic meters. Now we can plug ${\text{total quantity}=5}$ and ${\text{volume}=0.2}$ in the equation. $\begin{aligned} {\text{Density}}&=\dfrac{{\text{Total quantity}}}{{\text{Volume}}}\\\\ &=\dfrac{{5}}{{0.2}}\\\\ &=25 \end{aligned}$ Assuming Fatma fills the tank in its entirety, the density of the fish in the tank is $25$ fish per cubic meter.